1. Field of the Invention
This invention relates to the field of modulators for use in signal transmission systems.
2. Background Art
In electronic signal processing and data transmission, it is often desired to transmit data signals from a transmitting station to a receiving station. In practice, the data signal is modulated for transmission and demodulated on reception. Examples include radio wave transmissions, (including AM and FM) and telephone line transmission. The raw data which is to be transmitted, whether in analog or digital format, is known as a "base band signal". Typically, base band signals are not suitable for transmission in an efficient manner, if at all. In order to facilitate transmission of such information, base band signals must be shifted to high frequencies suitable for efficient transmission. This is done by varying the amplitude, phase or frequency (or combinations of these) of a high frequency carrier wave in accordance with the information to be transmitted. This is known as "modulation". In order to receive the information, it must be decoded by removing the carrier wave, this process being known as "demodulation".
Single units capable of both transmitting and receiving data corresponding include both a modulator and demodulator and are known as "modems". The use of modems has increased in recent years as computer use has increased. A modem allows data from one computer terminal to be transmitted through telephone lines and received by one or more computer terminals.
In the past, most modulators have been implemented in digital integrated circuit format. For example, digitized data is digitally modulated and fed to a multi-bit (8-10) ditital to analog (D/A) converter and the resulting signal is transmitted. Modems and associated modulators are typically controlled by a microprocessor under the command of a digital code. If the modulator is implemented in digital format, code space in the microcontroller must be dedicated to the modulator. In addition, in integrated circuit implementations, a digital to analog converter requires a large amount of silicon area, adding to the expense and size of the integrated circuit.
Further, in order to provide a high speed modem, it is desireable to define "symbols" representing multiple bits of data. One popular method of modulating data to provide such multi bit symbols is quadrature amplitude modulation (QAM). In quadrature amplitude modulation, a constellation of 16 points in defined which can be selected by four bits of information. Base band data is converted to two channels, known as an in-phase channel and a quadrature channel. Each channel represents two bits of information so that 16 points can be defined. Each channel must then be modulated and filtered prior to transmission. Thus, modems implementing a QAM scheme require two modulating channels, taking up additional silicon space.
In order to overcome these disadvantages, it is desired to provide a modulator implemented in complete analog format and which requires less silicon area to implement than digital modulators. It is further desired that a single analog modulator be used for modulation of two data channels.
Therefore, it is an object of the present invention to provide an analog modulator for modem applications.
It is another object of the present invention to provide an analog modulator which employs switched capacitor circuitry and requires less silicon area than a corresponding digital implementation.
It is yet another object of the present invention to provide an analog modulator which may be implemented with a single operational amplifier and be used with two data channels.